As is shown in FIG. 1 of the accompanying drawings, a motor vehicle headlamp conventionally includes a reflector 1 and a light source 2. The reflector 1 has a reflective base 4, for example of the parabolic type focussed in the region of the light source 2, together with an upper portion 5 and a lower portion 6, both of which are essentially flat and which extend from the top and bottom of the reflective base 4, respectively. Both the reflector 1 and the light source 2 are arranged inside a casing (not shown) that is open at one end, which open end is closed by a cover glass 3.
The upper portion 5 in some headlamps carries a metallic heat exchange plate 7, commonly called a radiator, the purpose of which is to prevent overheating of the headlamp. Conventionally, this radiator is reflective and diffusive, so that it retransmits some of the light which it receives from the light source 2 on to the lower portion 6. Where the latter is itself reflective or diffusive or both, then it will in its turn redirect the light in a beam LR, which is directed out of the headlamp as shown. This beam, resulting from double reflection, is a contributory factor to dazzle caused by the headlamp.
French patent document No. 2057246 discloses an optical unit for a motor vehicle, which is a headlamp of the general kind described above but in which the upper portion carries a striated element with Fresnel ribs. These ribs are parabolic and reflect outside the optical unit the light which they receive from the light source. The beam reflected in this way is a parallel beam having a high luminous intensity, which illuminates a portion of the road between the vehicle and that part of the road surface which is lit by the main beam from the optical unit. The optical unit proposed in French patent document No. 2057246 consequently eliminates that part of the zone which the driver usually sees as being in shadow in front of the vehicle.
Current thinking is that, for reasons of contrast, it is not desirable to illuminate this intermediate zone lying between the front of the vehicle and the zone lit by its main headlamp beams. Obstacles in the road will stand out better if this intermediate zone remains dark.